Autoclave furnace with mechanical circulation

ABSTRACT

An apparatus for treating a workpiece at elevated temperatures and pressures comprising an elongate cylindrical pressure vessel. Within the pressure vessel a hearth sits upon a pedestal. Surrounding the pedestal and the workspace immediately thereabove is an insulated furnace enclosure. A cavity near the base of the pedestal defines an impeller chamber. An impeller is positioned in the chamber and has a downwardly extending drive shaft. The impeller circulates the pressurized atmosphere. A remotely actuated gate directs the circulating atmosphere to either circulate totally within the furnace enclosure or partially within the furnace enclosure and partially along the interior wall of the pressure vessel.

There currently exist numerous uses for apparatus that treat a specimenor workpiece at high pressures and high temperatures including, forexample, gas pressure bonding furnaces and hot isostatic pressingapparatus. In these apparatus, it is typical to treat a workpiece at1000° C. and 15,000 psi although these are not the maximum temperatureand pressure conditions encountered. Suitable apparatus for theseapplications generally comprise a furnace within a pressure vessel orautoclave. The furnace provides the heat to the workpiece and protectsthe vessel from excessive temperature. The vessel maintains the furnaceand the workpiece at the desired pressures.

For a given pressure, the diameter of the pressure vessel determines theminimum safe thickness of the vessel wall. To avoid extremely heavyvessels, it is desirable to reduce the vessel diameter as much aspossible. Stated another way, the space between the interior of thevessel lining and the workpiece should be very small even though this isthe space occupied by the furnace.

In most processes, it is essential that the temperature of the workpiecebe extremely uniform. Otherwise, problems may result from differentialthermal expansion of the workpiece. Thus, the furnace portion of thehigh pressure-high temperature apparatus must distribute the heat evenlyto the workpiece.

It is an advantage of this invention to provide an autoclave or pressurevessel-furnace structure that minimizes the diameter of the pressurevessel, while at the same time providing for even distribution of heatto the workpiece in a way to obtain uniform workpiece temperature.

This application is related to our U.S. Pat. No. 4,151,400 whichdescribes mechanical circulation in an autoclave furnace.

SUMMARY OF THE INVENTION

Briefly, according to this invention, there is provided an apparatus forgas pressure bonding, hot isostatic pressing or the like in which aworkpiece may be treated at elevated temperatures and pressures. Theapparatus comprises an elongate cylindrical pressure vessel. Thepressure vessel further comprises an insulated hood or furnace forenclosing the workpiece and a hearth upon which the workpiece rests. Thehearth is set upon a refractory pedestal and a cylindrical heatingelement surrounds the pedestal preferably completely below the hearth.Preferably, the heating element is carbon or graphite. Other electricalresistance heating elements may be satisfactory including molybdenum ortungsten mesh. The heating element may be SiC for oxidizing atmospheresat lower power requirements. A cylindrical refractory shield may bedisposed about the pedestal and heating element in a way to permitconvection to transfer heat from the heating element to the workpieceplaced upon the hearth.

A cavity near the base of the pedestal forms an impeller housing havingradially extending exhaust ports opening out through the side of thepedestal. An impeller is positioned in the impeller chamber and has adownwardly extending shaft. Suitable means are provided for driving theshaft as by an electrical motor in the lower part of the pressurevessel. In one embodiment, the shaft passes through the bottom of thepressure vessel and enters a sealed drive unit where driven magnets aresecured thereto. Suitable magnet drives are those disclosed, forexample, in Ruyak U.S. Pat. Nos. 2,996,363 and 4,106,825.

The furnace is provided with a remotely actuated damper. Thus theimpeller may circulate the pressurized atmosphere only within thefurnace or by changing the damper position the impeller may circulatefurnace atmosphere along the interior of the pressure vessel walls aswell as within the furnace.

THE DRAWINGS

Further features and other objects and advantages of this invention willbecome clear from reading the following detailed description withreference to the drawings in which:

FIG. 1 is a section view through a furnace according to one embodimentof this invention,

FIG. 2 is a plan view in section corresponding to FIG. 1,

FIG. 3 is a schematic showing the circulation of furnace atmospheretotally within the furnace; and

FIG. 4 is a schematic illustrating flow of furnace atmosphere along theinterior of the vessel wall during cooling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a pressure vessel 1,2 arrangedoutside of a furnace comprising a hood 4, a shield 9, and heatingelements 8. A workpiece 7 is supported upon a hearth 6 and pedestal 5.Between the furnace hood 4 and the interior of the wall of the vessel 2is located a cylindrical baffle 3.

More specifically, referring to FIG. 1 there is shown a pressure vesselor autoclave comprising a base 1 and an inverted hat-shaped shell 2. Theflange at the base of the shell is provided with openings through whichfastening means enable the shell to be secured to the base. An O-ring orgasket 21 provides a pressure tight seal. The base or the shell isprovided with openings (not shown) which are connected to means forpressurizing the interior of the vessel, for example, with an inertatmosphere. Pressures up to 30,000 psi are typical. The thickness of theshell depends upon the pressures to be contained and the diameter of theshell. Typically the shell is made from high strength steel.

According to the invention, a pedestal 5 supported from the basesupports a hearth 6. The hearth should be strong enough to support theworkpiece at working temperatures. The hearth 6 has a diameter greaterthan the diameter of the pedestal. This enables the base of theworkpiece 7 to be greater than the top of the pedestal 5.

Preferably a hollow foot 22 supports a furnace bottom 23 somewhat abovethe base 1. The foot 22 and the furnace bottom 23 may be constructed ofcarbon steel. Setting upon the furnace bottom is heat and electricalinsulating support 24 which may be made from refractory insulting orhigh alumina castable. On top of the support 24 is an impeller chamberblock 25. A graphite, molybdenum or tungsten hearth 6 tops the pedestalextension 26. An anchor 27 fixed in the impeller chamber block engagesthe graphite pedestal extension to ensure alignment.

Surrounding the pedestal but not in contact therewith is a cylindricalcarbon or graphite, SiC or refractory metal (e.g., molybdenum)electrical resistance heating element. The heating element may comprisea cylindrical cage of rods 8 with adjacent rods forming pairs joined atthe top by caps 8a spanning each pair. Two conducting rings, one 8b withexternal teeth and another 8c with internal teeth are arranged aroundthe pedestal to form bases to support the cylindrical rods 8 and toprovide the pairs of rods with electrical current.

Electrical connecting means 35 and 36 are provided through the base ofthe vessel to supply an electrical current at an appropriate voltagelevel to the heating elements.

In the preferred embodiment, the furnace bottom 23, the insulatingsupport 24 and impeller chamber block 25 have openings therein to permitgraphite carbon, molybdenum or tungsten rods 29 threaded to theconducting rings 8b and 8c to pass into the space below the furnacebottom. Here means 30 couple the rods to the terminal 31 which isconnected to an electrical conduit passing through the base 1.

A refractory shield 9 is provided about the periphery of the heatingelement. Its principal function is to prevent radiation directly outwardfrom the heating element toward the hood 4.

The shield 9 may comprise an insulating refractory, say a lightweightinsulating brick or refractory castable. The shield may also comprise amulti-shell radiation shield. The shield 9 rests upon the furnace bottom24. The top of the shield must be vented. Holes 45 in the top of theshield are preferably centrally spaced.

Near the base of the hood 4 are two rows of openings. Each row ofopenings comprises a plurality of openings circumferentially spacedabout the hood. The centers of the openings in one row of openings liesubstantially in one plane parallel to the base of the vessel. The tworows of openings are axially or vertically spaced. Generally, the lowerrow of openings being return openings 62 are positioned radially outwardof the intake channels 47 explained hereafter. The higher row ofopenings being the exhaust openings 64 are spaced above the row ofreturn openings 62 and just above the insulating support 24.

The insulating support 24 has an axial opening 48 and intake channels 47extending radially therefrom. Channels 47 are arranged to align with thereturn openings 62 in hood 4. The impeller chamber block 25 has animpeller chamber with a plurality of radial exhaust ports 49 extendingtherefrom. An impeller 50 is positioned within the impeller chamber andis secured to a downwardly extending shaft 52. The shaft passes throughthe insulating support 24, the furnace bottom 23, the foot 20 and thebase 1 of the pressure vessel.

The shaft 52 passes into a sealed magnetic drive unit of the typedisclosed, for example, in U.S. Pat. Nos. 2,996,363 and 4,106,825. In analternate embodiment, the shaft 52 is driven by an electric motorpositioned between the base 1 and the furnace bottom 23. In thisinstance, the temperature of the space below the furnace bottom must becarefully maintained at a safe operating temperature for the electricmotor. Also, the electric motor requires the space between the bottomand the space to be enlarged.

Whatever means are used to turn the shaft 52, they should have avariable speed control. This will enable the circulation within thevessel to be tailored to the particular process or process stage takingplace within the vessel.

Where the shaft 52 passes through the furnace bottom 23 it may passthrough an axially aligned bushing in which the shaft is journaled. Thisis desirable where the length of the shaft, if not journaled whenpassing through the bottom 23, results in excessive vibrations. Theimpeller 50 may be of conventional "squirrel cage" design or any othersuitable design. Since the impeller will be subject to hightemperatures, it and shaft 52 must be made from materials that canwithstand such temperatures.

Up to this point, the structure described is generally similar to thatdescribed in our U.S. Pat. No. 4,151,400 except for the hood 3, the rowof intake openings 62 and the row of exhaust openings 64 in the hood.The tubular baffle 3 positioned between the interior of the wall of thevessel and the hood 4 has a plurality of openings 70 circumferentiallyspaced and also located radially outward of the intake channels 47 andreturn openings 62 in the hood.

A damper ring 72 slidable just within the baffle 3 is comprised of athin tube of refractory metal such as stainless steel or the like. Thedamper ring 72 has a radial flange 73 inwardly directed at the top edgethereof. The width of the radial flange 73 is sufficient to fill theannular space between the baffle 3 and the hood 4 with sufficientclearance for easy movement of the damper ring. The damper ring 72 has aplurality of openings 74 therein near the radial flange 73. The holesare arranged to be brought into alignment with the openings 70 in thebaffle 3 by updown positioning of the damper ring.

The baffle 3, like the damper ring 72, may comprise a thin tube ofrefractory metal as its only function is to direct the flow of thecirculating atmosphere.

A plurality of solenoid operated risers 76 are positioned on the base 1and arranged to raise and lower damper ring 72 a short distance, say 4-6inches, when electrically deactuated or actuated as the case may be.

At the uppermost position of the damper ring (as shown in FIGS. 1 and3), the furnace atmosphere is drawn down between the shield 9 and hood4, out the exhaust openings 64 in the hood and thence are directed bythe damper ring into the intake openings 62 in the hood to the intakechannels 47. At this time, the openings 70 in the baffle 3 and theopenings 74 in the damper ring are not aligned.

At the lowermost position of the damper ring (as shown in FIG. 4), thefurnace atmosphere is drawn down between the shield 9 and hood 4, outexhaust openings 64 in the hood. Then the atmosphere is directed byradial flange 73 up between the hood 4 and the baffle 3. Then it isdrawn down between the baffle 3 and the interior of the vessel wall 1.The atmosphere is then drawn through aligned openings 70 in the baffle 3and openings 74 in the damper ring 72 and into the intake opening 62 andintake channel 47.

When the impeller 50 is rotated it draws in the furnace atmosphere orgases along the impeller shaft 52 and forces the gases radially outwardinto the space between the pedestal wall and the shield 9 in thevicinity of the heating elements 8. The gases are heated passing by theheating elements (assuming they are heated at the time) and forced intothe space above the hearth 6 where they transfer heat to the workpiece.Thereafter, the gases pass between the shield 9 and the hood 4. Thereturn path then depends upon the position of the damper ring as alreadyexplained.

The insulating hood 4 is the principal heat insulation separating theworkpiece and the heating element from the pressure vessel shell. Thehood is designed to minimize heat transfer to the shell and to have alow heat capacity. A number of hood designs are possible. One shown inFIG. 1, comprises a stainless steel inner lining 40 and a carbon steelouter lining 41 with ceramic fiber heat insulation 42 therebetween.Other hood structures might comprise no inner sheet and refractoryinsulating brick in place of the fibers. An additional axial heat shield43 may be placed at the upper end of the hood for best results. Itshould be a refractory metal such as Inconel. As with the pedestal, theless heat energy absorbed by the hood, the more available for raisingthe temperature of the workpiece. Hence, the heat capacity of the hoodshould be minimized.

The heating element or elements according to this invention are locatedcompletely below the workpiece and thereby do not occupy space betweenthe workpiece 7 and the hood 4. This enables the diameter of the hoodand therefore the shell to be reduced with the advantages describedabove.

Having thus described the invention in the detail and with theparticularity required by the Patent Laws, what is desired protected byLetters Patent is set forth in the following claims.

We claim:
 1. An apparatus for gas pressure bonding, hot isostaticpressing or the like in which a workpiece may be treated at elevatedtemperatures and pressures, said apparatus comprising an elongatecylindrical pressure vessel for enclosing a furnace, a furnace bottomand an insulating hood resting upon the furnace bottom for enclosing theworkpiece and a hearth upon which the workpiece rests, a cylindricalbaffle positioned between the interior wall of the pressure vessel andthe insulating hood, an elongate cylindrical refractory pedestalextending upward from the furnace bottom, said hearth set upon saidrefractory pedestal, a cylindrical heating element disposed about andsubstantially along the entire length of said pedestal below saidhearth, a cylindrical shield disposed about the pedestal and heatingelement, said heat sheild extending up from the furnace bottom and abovethe hearth, said pedestal having an impeller chamber adjacent the basethereof with radial exhaust ports extending therefrom and an impellerpositioned in said chamber having a downwardly extending drive shaft,and remotely actuatable damper means for directing the flow of thefurnace atmosphere moving in response to the action of the impeller suchthat in a first position of the damper means flow is directed entirelywithin the furnace hood and in a second position flow is directed fromthe furnace interior to flow between the baffle and interior of thevessel wall prior to returning to the impeller.
 2. An apparatus for gaspressure bonding, hot isostatic pressing or the like in which aworkpiece may be treated at elevated temperatures and pressures, saidapparatus comprising an elongate cylindrical pressure vessel forenclosing a furnace, a furnace bottom, an insulating hood resting uponthe furnace bottom for enclosing the workpiece and a hearth upon whichthe workpiece rests, a cylindrical baffle having openings therethroughnear the bottom thereof, said baffle positioned between the interiorwall of the pressure vessel and the insulating hood, an insulatingsupport rests upon the furnace bottom having an axial opening and intakechannels extending radially therefrom, an elongate cylindricalrefractory pedestal having a base resting over the axial opening in theinsulating support, said hearth set upon said refractory pedestal, acylindrical heating element coaxial with said pedestal and resting onthe insulating base, a cylindrical shield disposed about the pedestaland heating element upwardly from the furnace bottom, said heat shieldextending up from the furnace bottom and above the hearth, said heatshield having openings therethrough near the bottom thereof, saidpedestal having an impeller chamber adjacent the base thereof and incommunication with the axial opening in the insulating support withradial exhaust channels extending therefrom and an impeller positionedin said chamber having a downwardly extending drive shaft, a remotelyactuatable damper means comprising a cylindrical shutter with a radialflange positioned between the hood and the baffle for directing flow ofthe furnace atmosphere moving in response to the action of the impellersuch that in a first position of the damper means the openings in thebaffle are closed off and flow is directed entirely within the furnacehood and in a second position the openings in the baffle are open andflow is directed from the furnace interior to flow between the baffleand interior of the vessel wall prior to returning to the impeller. 3.The apparatus according to claims 1 or 2 wherein solenoid actuateddevices move the damper means between first and second positions.
 4. Theapparatus according to claim 2, wherein the damper means comprises acylindrical portion and a radial flange attached thereto and means forremotely actuating the damper means to a first position with thecylindrical portion covering the holes in the baffle and the radialflange preventing flow upward between the baffle and hood as it emergesfrom the openings near the bottom of the hood and for remotely actuatingthe damper means to a second position with the cylindrical portionuncovering holes on the baffle and the radial flange preventing flowdownwardly between the baffle and hood as it emerges from openings nearthe bottom of the hood.